It is known from EP-A-378 994 that nitrobenzene synthesis wastewaters, which contain various aromatic compounds, in particular nitrophenols, can be purified solely by treatment with ozone at from 20 to 100.degree. C., a pressure of from 1.5 to 10 bar and a pH of from 3 to 12, essentially from 7 to 9, to such an extent that the levels of the original impurities are lowered to below their detection limits.
However, this method is not entirely satisfactory, since the ozone consumption is still relatively high and consequently, because of the high energy requirements of ozone generation, the costs are too high; that is, some of the ozone decomposes without contributing to the breaking down of the aromatics.